D-psicose as an epimer of a carbon-3 position of fructose has 70% sweetness as compared with sugar (Oshima 2006), but is functional monosaccharide applicable as a low-calorie sweetener of diet food in which energy is only 0.3% (Matsuo et al. 2002). Further, the D-psicose serves to inhibit glucose by suppressing absorption of glucose to be applied to food for diabetic patients, receiving food, and the like and serves to suppress the enzyme activity associated with lipid synthesis in the liver to suppress the accumulation of abdominal fat to be used in various functional foods such as health foods (Matsuo et al. 2001; Iida et al. 2008; Hayashi et al. 2010; Hossain et al. 2011).
As the above features, the psicose is a good source capable of replacing sugar, but the psicose belongs to a rare saccharide as a monosaccharide that is rarely present in the nature, and thus, a method for efficiently producing the psicose is required so as to be applied to food industry. The psicose is mostly produced through a chemical process as an existing method of producing the psicose. Bilik, et al. propose a method of converting fructose into psicose by using a catalytic action of molybdate ions. McDonald produced the psicose by a 3-step chemical treatment process from 1,2:4,5-di-δ-isopropylidene-beta-D-fructopyranose. Further, Doner produced the psicose by heating the fructose together with ethanol and trimethylamine. However, there are disadvantages in that a lot of costs are consumed in the chemical production methods, whereas the efficiencies thereof are low and a lot of byproducts are generated.
As a biological production method of psicose, a method of producing psicose from galactitol, D-tagatose, D-talitol, or the like by using a cell reaction of a microorganism is proposed (Ken Izumori). However, the method is difficult to be applied to the industrial production because the substrate is the rare saccharide. The most efficient method for industrialization is a method of finding an enzyme for converting fructose into psicose in a D-ketose 3-epimerase group. By the existing reported contents, the psicose was produced from the fructose by using a D-tagatose 3-epimerase which is expressed in E. coli transformed by inserting and transforming the D-tagatose 3-epimerase derived from Clostridium celluloticum H (10) (Mu et al. 2011), Agrobacterium tumefaciens (Kim et al. 2006), Pseudomonas cichorii (Itoh et al. 1994), Rhizobium spheroides (Zhang et al. 2009) in E. coli. With relation to a technique of producing psicose from fructose by using an enzyme, in Korea Patent Registration No. 10-0744479, a method of producing psicose by a psicose epimerase derived from Agrobacterium tumefaciens is disclosed, in Korea Patent Registration No. 10-0832339, Sinorhizobium YB-58 KCTC 10983BP having an activity of converting fructose into psicose and a method for converting fructose into psicose by using the same are disclosed, in Korea Patent Registration No. 10-1106253, E. coli including a polynucleotide coding a psicose 3-epimerase of Agrobacterium tumefaciens C58 having an activity of catalyzing conversion of fructose into psicose and a method of producing psicose from fructose by using the same are disclosed, in Korea Patent Registration No. 10-1339443 (Korea Patent Application Publication No. 10-2008-0071176), a ketose 3-epimerase derived from microorganisms belonging to genus Rhizobium and a method for converting fructose into psicose by using the same are disclosed, and in Korea Patent Registration No. 10-1318422, a D-psicose 3-epimerase derived from Clostridiuim scindens and a method for producing psicose from fructose by using the same are disclosed.
However, according to an existing enzymatic method of which functions are known, a method of producing psicose is best at a medium temperature and pH of an alkaline condition. In the alkaline condition, the reaction induces non-specific reaction and browning of sugar, and thus is not suitable for the industrialization. Further, there is a problem in that the existing enzymes have factors of increasing producing costs for producing psicose which is applied to the industrialization due to deteriorated stability or a slow reaction rate at a high temperature. Therefore, development of a novel D-psicose 3-epimerase in which a product yield, a temperature, a pH and a reaction rate of the psicose are suitable for industrialization is required. With relation to this, in Korea Patent Application Publication No. 10-2014-0021974, a D-psicose 3-epimerase derived from Treponema primitia ZAS-1 having a rapid psicose conversion rate and stability at a high temperature by inducing mutation at a gene level is disclosed, and in Korea Patent Registration No. 10-1203856, psicose epimerase variants having improved thermal stability obtained by mutation of a wild-type psicose epimerase derived from Agrobacterium tumefaciens is disclosed.